Sensors for the detection of particular compounds present in a high temperature gas stream find numerous applications in many different mechanical systems. For example, detection of certain compounds in a high temperature gas stream is important in industrial emission monitoring for detection of gas pollutants, such as sulfur dioxide (SO.sub.2), in residential heating systems for detection of carbon monoxide (CO), and in automobile exhaust systems for various compounds including hydrocarbons.
In automotive applications, gas sensors can be placed at various locations in an exhaust system. Exhaust gas from an internal combustion engine typically contains hydrogen (H.sub.2), carbon monoxide (CO), methane (CH.sub.4), carbon dioxide (CO.sub.2), nitric oxide (NO), water (H.sub.2 O), and nonmethane hydrocarbons (C.sub.n H.sub.m), where n is an integer larger than 1 and m is an integer whose value depends upon the kind of hydrocarbon compound, for example, alkane, alkene, alkyl, or aryl. Important environmental pollution concerns dictate that the emission of hydrocarbons be minimized. To minimize pollutants in the engine exhaust, sensors can be placed before and after the catalytic converter to monitor the performance of the converter. Also, the emission of hydrocarbons can be controlled, in part, by an engine exhaust control system that receives a feedback signal from an exhaust sensor capable of selectively detecting the presence of hydrocarbons in the engine exhaust.
One method for monitoring the performance of a catalytic converter includes the use of oxygen sensors within the exhaust gas system. By measuring the amount of oxygen in the exhaust gas entering and exiting a catalytic converter, an estimate of the amount of oxygen stored in the catalytic converter can be made. Since oxygen storage capacity is necessary for a catalytic converter to oxidize hydrocarbons and reduce nitrogen oxide, an indirect measurement of the pollution control efficiency of the catalytic converter can be made by determining the amount of oxygen stored within the catalytic converter at any given point in time. Accordingly, by estimating the amount of oxygen stored within the catalytic converter, an indirect measurement of the catalytic converter efficiency can be obtained. Although oxygen sensors are relatively simple to manufacture, using oxygen measurements to estimate catalytic converter performance is imprecise.
A sensor that directly estimates the hydrocarbon concentration in an exhaust gas stream can be used to provide a more precise determination of catalytic converter efficiency. For example, several types of sensing elements have been developed for detecting various chemical species within an exhaust gas stream. These sensing elements includes calorimetric sensors having a catalyst coating, semiconductor metal oxide based sensors, and the like. Calorimetric hydrocarbon gas sensors measure the amount of heat released by the catalytic oxidation of hydrocarbons contained within the exhaust gas. To obtain optimum sensitivity for the measurement of hydrocarbon species within an exhaust gas, a calorimetric hydrocarbon gas sensor must be designed to maintain a relatively constant internal temperature. This requirement is especially important given the wide temperature variations encountered in an exhaust gas system, together with the need to internally generate oxygen for catalytic combustion within the sensor.
While providing a measurement of hydrocarbon concentration, a calorimetric hydrocarbon gas sensor must be carefully designed for operation in a high temperature exhaust gas stream. For precise measurement of hydrocarbons in an exhaust gas, small temperature rises, or small quantities of liberated heat, must be detected when the hydrocarbons are oxidized within the sensor. Detection these small variations can be difficult when exhaust gas temperatures are rapidly changing and subjecting the sensor to a variable temperature environment. For example, automotive engine operation is dynamic and the exhaust gas temperature varies from ambient temperature, at engine start-up to more than 1,000.degree. C. during periods of high power operation. Thus, in calorimetric hydrocarbon gas sensor technology for applications to high temperature exhaust gas systems, a major technical challenge involves thermal management within the gas sensor.
In addition to the need to accommodate thermal variations within the exhaust gas, calorimetric sensors require an oxygen source for the catalytic oxidation of the hydrocarbons. Typically, the oxygen supply systems used in calorimetric hydrocarbon gas sensors must operate at elevated temperatures. High temperature operation is necessary to attain the level of efficiency needed to supply sufficient oxygen to the catalyst within the sensor. The necessity of including an oxygen supply system adds additional design constraints for a calorimetric hydrocarbon gas sensor. Thus, improved thermal management is needed within a calorimetric hydrocarbon gas sensor designed for the measurement of hydrocarbons in an exhaust gas stream.